This invention is directed to a method of substantially completely catalytically oxidizing methanol vapor and/or formaldehyde vapor contained in a heated carrier gas and, more particularly, to a method of treating exhaust gases generated by burning a methanol fuel in an internal combustion engine.
2. Description of the Prior Art
Catalytic oxidation of methanol has been the subject of a number of laboratory investigations mainly concerned with the oxidation of methanol to formaldehyde and not by an oxidation method which produces little or no formaldehyde. See Dixon, J.K. and Longfield, J.E., Catalysis 7, pages 183, 231-236, Rheinhold, New York (1960); Edwards, J., Journal of Catalysis, Vol. 50, page 24 (1977); Santacesaria, E. and Marbidelli, M., Chemical Engineering Science, Vol. 36, page 909 (1981).
More recently the interest in methanol as a viable alternative fuel for automobiles has led to the search for catalysts which completely oxidize methanol vapor with little or no production of aldehydes in the purification of exhaust gases from methanol-fueled vehicles. The untreated exhaust gas contains, among other oxidation products, appreciable amounts of formaldehyde and unburned methanol fuel, especially during the engine warm-up period. Since formaldehyde, known as an eye irritant and a potential carcinogen, is an intermediate product from the oxidation of methanol, its emissions from methanol-fueled vehicles desirably should be minimized by catalysts highly effective for the complete oxidation of any unburned methanol in the exhaust. Such catalysts would also be efficient for the oxidation of the formaldehyde present in the untreated exhaust gases.
A previous study at Ford Motor Company of potential oxidation catalysts for the treatment of methanol-fueled engine exhaust gases (see U.S. Pat. No. 4,304,761) concluded that the base metal silver alone would completely oxidize methanol, at low concentrations and in the presence of excess oxygen (lean mixtures) to carbon dioxide and water vapor without production of deleterious amounts of aldehydes (3% or less), ethers or carbon monoxide. (More recent concensus concerning nonregulated automotive emissions indicates that 3% aldehydes may not be acceptable.) The study used a laboratory simulation of the exhaust gas from an engine that would utilize a lean air/fuel mixture to yield an exhaust gas containing one percent excess oxygen. The results of the study of both the treated silver catalYst and other catalysts for oxidizing methanol (all pretreated by heating at 800.degree. C. for six hours) were obtained at a space velocity of 300,000 Hr.sup.-1. The 4,304,761 patent is based on results for granular catalytic support material (i.e., without monolithic substrate). This space velocity should not be compared with the space velocities for monolithic catalyst supports. These results show that certain catalysts, such as palladium and rhodium, did not function as well as the silver catalyst, and such patent reports that these materials have an undesirable affinity for producing aldehydes (see the aforesaid patent, column 3, lines 53-55). There was no investigation of how silver, or any of the other catalysts deployed would perform in the presence of little or no oxygen or at conditions (i.e., space velocity) typical of exhaust gases issuing from an internal combustion engine.
Most of the internal combustion engines for automobiles of today, and for the foreseeable future as well, are designed for operation under stoichiometric conditions with exhaust gases containing little or no excess oxygen. Moreover, state-of-the-art technology for closed-loop feedback control of the air/fuel ratio is such that, occasionally, a momentarily fuel-rich mixture will enter the engine followed by a momentarily fuel-lean mixture. Thus, a preferred catalyst system for the exhaust gas treatment should be capable of achieving high methanol conversion (greater than 96%) with minimal formaldehyde formation for exhaust gas which while containing little or no excess oxygen over a finite period, momentarily may be rich or deficient in excess oxygen.
It is therefore a primary object of this invention to provide an efficient method of catalytically oxidizing methanol vapor in a carrier gas with little or no production of aldehydes, the carrier gas containing methanol in a low concentration, i.e., 0.01-1.0%, and containing little or no excess oxygen.
It is also an object of this invention to provide a method of catalytically oxidizing methanol vapor contained in the exhaust gas of an internal combustion engine designed for operation with substantially stoichiometric air/fuel mixtures, the oxidation process achieving 96% or greater conversion of such methanol to CO.sub.2 and water vapor aldehyde formation below 1% of the converted methanol.